Specifically, the photocatalytic self-cleaning products can decompose fouling by ultraviolet light present in the spectrum of sunlight and also wash off fouling on surfaces of the products by the super-hydrophilic property when rain falls. Therefore, the products are able to constantly maintain clean appearance (self-cleaning function).
There are several problems concerning photocatalyst coated self-cleaning glass among the photocatalytic self-cleaning products. In general, photocatalyst coated self-cleaning glass is produced by coating a surface of the glass with particles of titanium oxide (TiO2). Since a contact area between the titanium oxide particles and the glass surface is small, a coating layer is easily worn off. Adhesion between the coating layer and the glass can be improved if firing is conducted at high temperature of several hundreds centigrade after the coating layer is formed. In that case, however, photocatalytic activity is decreased. Specifically, since soda-lime glass that is widely used for a window contains a considerable amount of sodium ion, the sodium ion is diffused into the coating layer at the surface of the glass during the firing. As a result, a compound of titanium oxide and sodium (like sodium titanate) is formed and the photocatalytic property of the glass is lost.
To avoid such problem of photocatalytic performance loss due to alkali diffusion, two methods are taken in manufacturing of photocatalytic self-cleaning glass. One of the methods is called a room temperature curing method. In the method, fine particles of titanium oxide photocatalysts are mixed into a coating liquid that contains sol-gel components and is solidified at comparatively low temperature. The resulted mixture is applied to glass and solidified at around 150° C. (see Patent Document 1, for example). According to this method, as shown in FIG. 2, a photocatalytic film 105 in which fine particles 103 of titanium oxide is diffused in a sol-gel film 101 is formed on a surface of a glass base 107.
The other of the methods is called a two-layer coating method. In this method, as shown in FIG. 3, an undercoating film 203 containing a component like silica (SiO2) is formed on a surface of a glass base 201 in order to inhibit alkali diffusion from the glass base 201 due to firing. Thereafter, photocatalytic coating is performed to form and then fire a photocatalytic film 205 (see Patent Document 2, for example).
A similar problem exists in glazed ceramic products and enameled metal products. Glaze and enamel are liquid glasses which are melted or dissolved in a medium. Since glaze and enamel includes a massive amount of sodium components in order to lower a melting point thereof, the room temperature curing method or the two-layer coating method has to be used in order to form a photocatalytic film on a surface of these products.
Patent Document 1: Unexamined Japanese Patent Publication No. 2001-150586
Patent Document 2: Unexamined Japanese Patent Publication No. 10-53439
In the room temperature curing method, since adhesion between glass and a coating film is not high, abrasion resistance of the film is low. This method can only be adopted for very limited use, such as in a part where there is little physical contact with others (like a window of a tall building, for example).
In the two-layer coating method, coating has to be performed twice. In addition, according to circumstances, the undercoating film also requires firing. In that case, firing steps have to be conducted twice. Cumbersome steps and high costs become necessary.